A flyback converter is a transformer-isolated converter based on the basic buck-boost topology. In a flyback converter, a switch is connected in series with the transformer primary side. The transformer is used to store energy during ON periods of the primary switch, and provides isolation between the input voltage source and the output voltage. In a steady state of operation, when the primary switch is ON for a period of TON, during the TON period, a diode on the secondary side becomes reverse-biased and the transformer behaves as an inductor. The value of this inductor is equal to the transformer primary magnetizing inductance LM, and the stored magnetizing energy from the input voltage source. As such, the current in the primary transformer (magnetizing current IM) rises linearly from an initial value to a peak value. As the diode on the secondary side becomes reverse-biased, the load current is supplied from an output capacitor on the secondary side. The output capacitor value is ideally large enough to supply the load current for the time period TON, with the maximum specified drop in output voltage.
To increase system efficiency, flyback converters typically use Synchronous Rectification (SR) controller and a secondary-side SR power MOSFET. The secondary-side SR power MOSFET is turned on and off synchronously with the primary side power MOSFET. Some conventional secondary-side controllers have an SR sense pin used for voltage sensing to turn off the SR power MOSFET on the secondary side, and which has a very high breakdown voltage requirement (e.g. up to 120V or even higher), so the chip technology used to implement the secondary-side controller must support very high voltages. The SR sense pin is used for voltage sensing, which has a very low negative threshold voltage comparison requirement (e.g. around −10 mV with 10 uV accuracy), which is very difficult to implement in standard chip technologies. Other conventional secondary-side controllers do not require high voltage technology for the controller and do not need to compare against a very low negative threshold voltage for detecting when to turn off the secondary-side SR power MOSFET. However, these controllers suffer from settling time variation which causes the measurement of the reflected input voltage from the secondary side of the transformer to have some error, especially for high frequency and high input line cases. This variation greatly influences the calculation of the turn-on timing for the secondary-side SR power MOSFET. Errors in the SR on-time calculations is problematic, and leads to inefficient operation. Accordingly, conventional secondary-side controllers are designed for applications operating over a relatively narrow operating range. Improved secondary-side controllers and SR control techniques are therefore desired.